Reynolds portrait emerges from 200 years of grime

'Lady Talbot', unseen in public since 1782, is to go on show again. Anthony Barnes reports

A work by the master portrait painter Joshua Reynolds that has not been displayed for more than 200 yearsis to go on show after lying hidden in a vault for decades.

A work by the master portrait painter Joshua Reynolds that has not been displayed for more than 200 yearsis to go on show after lying hidden in a vault for decades.

Lady Talbot, which has not been seen by the public since it debuted at the Royal Academy in 1782, had been branded "unexhibitable" as it lay obscured beneath a blanket of disfiguring grime. This week, after a painstaking three-year operation to remove layer after layer of discoloured varnish, it is to go on show at Tate Britain.

"It's almost like acquiring a new painting. We have had it but nobody has seen it. It's a tour de force," said Dr Martin Postle, the Tate's head of British art to 1900.

Reynolds was the David Bailey of his day, capturing the likeness of the prominent figures of 18th-century British life. His trick would be to paint them just as they were on the cusp of fame. His subject's stock would then rise and Reynolds would gain kudos for being linked with the latest big name.

His models included his friend David Garrick, Edmund Burke and Samuel Johnson, as well as debutantes, the gentry, military officials and intellectuals.

"There was this kind of interdependence," said Dr Postle. "He was a fantastically skilled networker; he'd get to know everybody. It's become apparent to me that fame was very important to him and his reputation. He had a very wide circle of acquaintances, and his own fame and success were built to some extent on the people he painted."

Dr Postle had wanted to include the painting in a new exhibition, Joshua Reynolds: The Creation of Celebrity, if it could be salvaged, as an example of a key series of works the artist executed of society beauties.

The best known of these in the Tate's collection is a portrait of Lady Bampfylde, which also forms part of the exhibition.

Lady Charlotte Talbot, the daughter of the Marquess of Downshire, was painted in Grecian dress, pouring a sacrificial offering to a statue of the goddess Minerva - a formula he had used many times. Her portrait, which originally cost 200 guineas and measures 8ft by 5ft, had been in private hands until it was bequeathed to the nation in 1941 by Sir Otto Beit.

Joshua Reynolds: The Creation of Celebrity is at Tate Britain from Thursday to 18 September

Rolling back the years, layer by layer

* Treatment starts with the careful removal of dirt and grime from the surface, using water.

* A tiny area is tested to ensure the appropriate organic solvent is used.

* Then the painstaking cleaning process begins, wiping away the layers of varnish. This may have to be repeated a number of times.

* Finally a protective, non-yellowing, stable synthetic resin varnish is applied.